Audiovocal communication for maintaining maternal-offspring contact counts as one of three key developments in the evolutionary transition from reptiles to mammals. The isolation (separation) call putatively ranks as the most primitive and basic mammalian vocalization, serving originally to maintain maternal-offspring contact. Previous work on this project utilizing the squirrel monkeys (Saimiri sciureus) has indicated that the isolation call is represented in an as yet undefined band of medial frontal limbic cortex and contiguous neocortex. In the continuation of this study an effort is being made to identify thalamic nuclei and related frontal cortical areas involved in the production of spontaneous isolation calls. Ablation of the frontal lobes rostral to the genu of the corpus callosum, but not an ablation forward of the cingulate sulcus, results in an enduring failure to emit spontaneous calls. Aspiration of the frontal limbic cortex of the anterior cingulate, pregenual, and preseptal areas, along with part of "area 32," appears to be sufficient to eliminate spontaneous calls. Ablation of the face area of the supplementary area results in a transitory failure to produce calls. The anterior medial thalamic (AM) and parts of the medial dorsal nucleus (MD) have overlapping projections to parts of the frontal limbic areas. Current findings indicate that destruction of AM is not sufficient to eliminate spontaneous calls, and that additional loss of overlapping projections from MD may be necessary to obtain the deficit.